The present invention generally relates to a display device and more particularly, to a method of driving a display device, for example, a capacitive flat matrix display panel (referred to as a thin film EL display device hereinafter), etc.
FIG. 8 is a block diagram showing overall construction of a conventional thin film EL display device, which generally includes a display panel 1, a data side switching circuit 2, a scanning side switching circuit 5, a drive circuit 8, and a drive logic circuit 11 coupled to each other as illustrated.
In the known arrangement of FIG. 8, the display panel 1 is composed of a thin film EL element. In the case where this thin film EL element is, for example, of a double insulated type thin film EL element, it includes belt-like transparent electrodes arranged in a parallel relation on a glass substrate, a dielectric substance applied thereover, an EL layer further applied thereover, and another dielectric substance further applied thereon to provide a three-layered structure, and belt-like back electrodes further applied thereover so as to extend parallely in a direction intersecting at right angles with said transparent electrodes referred to above.
In the display panel 1 as described above, the transparent electrodes of the thin film EL element are set to be the data side electrodes x1 to x8, while the back electrodes of said thin film EL element are adapted to be the scanning side electrodes y1 to y4.
The data side switching circuit 2 is intended to apply OV or V.sub.M as a modulation voltage individually to the respective data side electrodes x1 to x8, and includes a group of data side output ports 3 individually connected to the respective data side electrodes x1 to x8, and a logic circuit 4 which receives display data corresponding t the respective data side electrodes x1 to x8 so as to turn on or off the data side output ports 3 according to said display data.
Meanwhile, the scanning side switching circuit 5 is a circuit for impressing Vw1 or -Vw2 (in a relation Vw1=Vw2+V.sub.M, and represented as Vw1.gtoreq.Vth, Vw2.ltoreq.Vth when light emitting threshold voltage of the thin film EL element is denoted by Vth) to the respective scanning side electrodes y1 to y4 according to the line sequence thereof as a writing voltage, and includes a group of scanning side output ports 6 individually connected to the respective scanning side electrodes y1 to y4, and a logic circuit 7 for turning on or off the group of the scanning side output ports 6 according to the line sequence of the scanning side electrodes y1 to y4.
The drive circuit 8 is arranged to generate a high voltage for driving the display panel 1 from a predetermined constant reference voltage V.sub.D, and is provided with a modulation drive circuit 9 for supplying modulation voltage Vm to the data side output ports 3, and a write drive circuit 10 for supplying write voltages Vw1 and -Vw2 to the scanning side output ports 6.
The drive logic circuit 11 is a circuit for generating various timing signals necessary for driving the display panel 1, based on input signals such as a display data signal D, a data transfer clock CK, a horizontal synchronizing signal H, and a vertical synchronizing signal V, etc.
The fundamental driving for the display of the thin film EL display device as described above is effected by applying OV or V.sub.M to the data side electrodes x1 to x8 as modulation voltages corresponding to the display data which determine light emission or non-light emission, with a section extending over first and second two frames being set as one period, and also, by applying the write voltage Vw1 to the scanning side electrodes y1 to y4 at the first frame, and the write voltage, -Vw2 thereto at the second frame by line sequence.
By the above display function, a superposing effect or offset effect of the write voltage Vw1 or -Vw2 and the modulation voltage OV or V.sub.M is produced at the portions of picture elements where the data side electrodes x1 to x8 and the scanning side electrodes y1 to y4 intersect each other, and a voltage Vw1 higher than a light emitting threshold voltage Vth or a voltage Vw2 lower than the light emitting threshold voltage Vth is applied to each picture element as an effective voltage, whereby the respective picture elements are brought into the light emitting state or non-light emitting state to provide the predetermined display. Accordingly, with respect to one image element, effective voltage inverted in its polarity between the first frame and the second frame respectively is alternately impressed, and thus, with the two frames set as one period, symmetrical A.C. driving ideal for a thin film EL element is to be effected.
Conventionally, in the thin film EL display device as described above, as a driving method for varying brightness of the respective picture elements in a plurality of stages, i.e. for effecting gradation display, there have been known an amplitude control system for controlling amplitude of the modulation voltage V.sub.M to be impressed to the data side electrodes x1 to x8, a pulse width modulation system for varying pulse width of the modulation voltage V.sub.M, and a frequency modulation system for thinning out the display data of either the first frame or second frame.
However, in the driving methods of the amplitude modulation system or pulse width modulation system as described above, there has been for that a problem that, if it is intended to increase the number of stages in the gradation, the amplitude or pulse width of the modulation voltage V.sub.M must be controlled very finely during one scanning period, and in order to effect such a control at a high accuracy, circuit construction is undesirably complicated, thus resulting in cost rise.
Furthermore, when the gradation display is to be effected by the driving method of the known frequency modulation system as referred to above, there has also been such a disadvantage that, in the case where the display data is cut in the first frame or in the second frame, the effective voltages to be applied to the picture element is the same in the one period consisting of the first frame and second frame, and as a result, all, the gradations which can be displayed are limited only to three gradations namely, light emission, non-light emission and intermediate tone, and also with limitation to the increase in the number of stages for the gradations.